From b5fa4b90a7a1a9552302bbc83e07024c127345bc Mon Sep 17 00:00:00 2001
From: Aurimas Liutikas <aurimas@google.com>
Date: Wed, 11 Dec 2024 14:47:59 -0800
Subject: [PATCH] Add a post about artifact aggregation

---
 Gemfile.lock                               |  2 +-
 _posts/2024-12-11-Together-In-Isolation.md | 91 ++++++++++++++++++++++
 2 files changed, 92 insertions(+), 1 deletion(-)
 create mode 100644 _posts/2024-12-11-Together-In-Isolation.md

diff --git a/Gemfile.lock b/Gemfile.lock
index bbfd7c6..b5e696a 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -274,7 +274,7 @@ DEPENDENCIES
   github-pages
   jekyll-feed
   tzinfo-data
-  webrick (~> 1.8.1)
+  webrick (~> 1.8.2)
 
 BUNDLED WITH
    2.4.7
diff --git a/_posts/2024-12-11-Together-In-Isolation.md b/_posts/2024-12-11-Together-In-Isolation.md
new file mode 100644
index 0000000..6aa5c95
--- /dev/null
+++ b/_posts/2024-12-11-Together-In-Isolation.md
@@ -0,0 +1,91 @@
+---
+layout: post
+title:  Together in Isolation - Isolated Project Safe Way to Aggregate Optional Artifacts 
+header: header-eng
+---
+
+One common thing to do in a Gradle project is collect artifacts from a subset of projects to produce an aggregate
+artifact. An example of it might be collecting test reports from each project that have tests to create a combined
+test report for all projects. Historically, someone might have done this with:
+
+Root project `build.gradle.kts`:
+```kotlin
+abstract class AggregatingTask : DefaultTask() {
+    @get:InputFiles
+    abstract val inputs: ListProperty<File>
+    @get:OutputFile
+    abstract val output: RegularFile
+    @TaskAction fun aggregate() { /* process inputs */ }
+}
+
+tasks.register<AggregatingTask>("aggregatingTask")
+```
+
+Each project that has an artifact in their `build.gradle.kts`:
+```kotlin
+rootProject.tasks.named<AggregatingTask>(
+    "aggregatingTask"
+).configure { 
+    inputs.add(reportTask.flatmap { it.reportFile })
+}
+```
+
+Sadly, in addition to being fragile with hardcoding of the task name/type, it is also not valid with Gradle
+[Isolated Project](https://docs.gradle.org/current/userguide/isolated_projects.html) feature enabled. Is there a better
+way? Yes (but it is a tad verbose)!
+
+Root project `build.gradle.kts`:
+```kotlin
+val reportConsumer = configurations.create("reportConsumer") {
+    isCanBeResolved = true
+    isCanBeConsumed = false
+    attributes {
+        attribute(
+            Category.CATEGORY_ATTRIBUTE,
+            objects.named("my-reports")
+        )
+    }
+}
+// Add all subprojects to dependencies
+subprojects {
+    reportConsumer.dependencies.add(dependencies.create(this))
+}
+// Be lenient if not all projects have this artifact
+val reportCollection = reportConsumer.incoming.artifactView {
+    lenient(true)
+}.files
+
+abstract class AggregatingTask : DefaultTask() {
+    @get:InputFiles
+    abstract val inputs: ConfigurableFileCollection
+    @get:OutputFile
+    abstract val output: RegularFile
+    @TaskAction fun aggregate() { /* process inputs */ }
+}
+tasks.register<AggregatingTask>("aggregatingTask") {
+    inputs.from(reportCollection)
+}
+
+```
+
+Each project that has an artifact in their `build.gradle.kts`:
+```kotlin
+configurations.create("reportPublisher") {
+    // Note, consumed and resolved are opposite of root project
+    isCanBeResolved = false
+    isCanBeConsumed = true
+    attributes {
+        attribute(
+            Category.CATEGORY_ATTRIBUTE,
+            objects.named("my-reports")
+        )
+    }
+}
+artifacts.add("reportPublisher", reportTask)
+```
+
+With this set up, root project still has the aggregating task that is able to build the report from all the artifacts
+from each project, but instead of each subproject contributing to that task directly, everything flows through Gradle
+artifact publishing and attribute resolution mechanisms.
+
+Pro-tip: to visualize what you have added to artifacts, you can use `./gradlew :projectA:outgoingVariants`